Synchronizing signal generator



Feb. 29, 1944. H. c. RESSLER SYNCHRONIZING SIGNAL GENERATOR Filed Oct. 25, 1940 A INVENTOR. ji'zyfp 6 Hassle? By I ATTORNEY Patented Feb. 29, 1944 SYNCHRONIZING SIGNAL GENERATOR Hugh 0. Ressler, West Brighton, Staten Island, N. Y., assignor to Faximile, Inc., New York, N. Y., a corporation of Delaware Application October 25, 1940, Serial No. 362,775

Claims. (01. 178-695) The present invention concerns synchronizing and, in particular, the generation of synchronizing signals in a facsimile transmitter or the like.

One object of the present invention is to provide a method of and means for generating a synchronizing impulse of improved characteristics.

A further object is to provide simple means for generating a highly accurate synchronizing signal.

A still further object is to provide a method of and means for switching a synchronizing signal into a signal channel without altering in an way the shape or duration of the signal.

These and other objects will be apparent from the detailed description of the invention given in connection with the various figures of the drawing.

In the drawing:

Fig. 1 shows a facsimile transmitter circuit embodying one form of the present invention.

Fig. 2 shows a diagram useful in explaining the operation of the invention as shown in Fig. 1.

Fig. 3 shows a second diagram useful in explaining the operation of the invention as shown in Fig. 1.

Fig. 4 shows a third diagram illustrating the operation of the invention.

The transmission of facsimile by wire or radio is generally accomplished by scanning a subject to be transmitted line by line by means of a point of light, picking up the reflected light from the subject in a photo-electric cell and modu lating a sub-carrier signal with ,the amplified output of the photo-electric cell to form a modulated picture signal. This modulated picture signal may be transmitted over wire lines to a receiving point, or it.may be used to modulate a radio frequency carrier which is then radiated, to be picked up at the receiver. At the receiver the modulated signal is rectified and the rectified current is utilized to actuate a, recorder to reproduce the facsimile image.

In the process just described, it is important that the transmitting and receiving scanners opchronizing signal. The accuracy in time at which this impulse starts and/or ends determines the accuracy with which the receiver may be timed when utilizing this signal.

In the past this synchronizing impulse has been produced by closing an electrical circuit between the source of sub-carrier signal and the transmission circuit for an interval equal to the desired duration of the synchronizing impulse. A commutator segment attached to the transmitter scanner drum shaft, a cam operated switch or a relay has been used to close the electrical circuit. It has been found that a high degree of accuracy is difficult to obtain by any of these prior means. It has also been found that since these means close current carrying circuits, transients are produced which disturb the system and detract from the accuracy of synchronization.

The present invention concerns a system in which highly accurate timing impulses are produced by electronic means. These impulses are so formed that they may be switched into and out of the transmission circuit when no current is flowing in the source. In this way the accuracy of the synchronizing impulses may be predetermined and it is not affected by the switching operation. In addition, the switching at zero current prevents the production of tran-- sients.

Fig. 1 shows a facsimile transmitter embodying one form of the present invention. Fig. 1 shows a 60 cycle synchronous motor 4 connected to a source of 60 cycle power by means of leads 9 and Ill. Motor 4 drives transmitter pick-up drum 6 thru a suitable reducing gear box 5. Gear box 5 may, for instance, transform a motor shaft speed of 3600 revolutions per minute to a drum shaft speed of 200 revolutions per minute, a reduction of 18 to 1. Drum 6 carries a commutator I provided with shorting bar 8 grounded by brush 2 in contact with conducting ring 8' also connected to bar 8. Brushes l and 3 are grounded whenever they contact bar 8. Subject matter on the surface of drum 6 is scanned by directing a spot of light from lamp filament ll, thru lens I2 to the subject matter, and'picking up the reflected light by means of lens i3, directed to photoelectric cell The output of photo-electric cell 14 is fed to the amplifier and sub-carrier modulator IS. A source of sub-carrier signals I9 is also fed to amplifier-modulator I5 and the output of this amplifier-modulator I 5 is fed to the radio modulator l6 from which the final signals are fed to antenna i1 and ground l8.

In order to generate synchronizing impulses,

means of a transformer having primary 2!! connected to the 60 cycle power source and secondary 2! connected to diode 28 and secondary 22 connected to diode 23. One side of secondary 2i is connected to cathode 24 of diode 23 and the other side is connected to one end of diode load resistor 25. Anode 25 of diode 23 is connected to the otherend of diode loadresistor 26. During the negative part of the cycle of the 60 cycle input, diode 23 will pass current, making the anode end of resistor 26 negative with respect to its other end. The anode end of resistor 26 is connected to grid 32 of triode 30, and the other end to cathode 3!. Plate 33 is connected to a, source of plate potential 53. During the intervals of diode current flow, grid 32 receives a negative bias with respect to cathode 3i and the cathode current of the tube 39 is cutoif. During the other half cycle of the 60 cycle wave, no current is passed by diode 23 and a steady cathode current flows in tube 30. Hence, across resistor 38, connected between cathode 3i and ground It, will appear rectangular voltage impulses corresponding to positive half cycles of the 60 cycle current, as shown at a in Fig. 2.

In a similar manner but with reversed polarity,

diode 23' passes current from secondary 22 from of curves a and b is due to bias from either resistor 35 or 3d feeding back to the. other thru the isolating resistors tit andili. The sum of a and b of Fig. 2, as shown at m in Fig. 3, is applied to cathodes it and 69 of diodes 55 and Q8 thru isolating resistors at and ti, lead M and secondary 33. As long as cathodes t6 and 39 receive a positlve bias of more than a critical value, as shown by dotted lines, t, a: and y, they will not pass current. Sub-carrier voltage is applied to cathodes 66 and 39 by means of transformer 32-443, but it is passed to plates ll and 56, primary 5|, secondary 52 and thus to radio modulator I 6 only when the positive bias is removed from cathodes it and :39. This bias is removed during predetermined intervals, in order to pass impulses of sub-carrier signals to modulator B6, to provide synchronizing signals. Brush i grounds at the 2,842,693 60 cycle current is fed to diodes 23 and 23' by 45 and 68. At a, diodes l5 and 48 are again at .out-ofl,-due to the drop across resistor 39. It will be seen that an interval of zero bias extending from h to k in Fig. 2 is produced and diodes l5 and 48 are permitted to pass an impulse of subcarrier voltage for this interval. It will also be seen that the duration of this impulse is accurately determined by the 60 cycle half wave impulses and is not affected by the time at which brushes l and 3 close and open. Thus, an electrical impulse is produced for synchronizing purposes, the duration of which is electrically determined, even' though it is mechanically switched. The accuracy of the switch operation does not aflect the duration of the impulse. The

. width and angle of commutator segment a may instant it touches bar 3, shorting resistor 39 and dropping its voltage to zero. This is shown where curve 12 of Fig. 2 intersects vertical line 0. Tubes 35 and at are at cut-off when this shorting takes place, due to the positive bias from resistor 39, as shown by the intersection of curve a and vertical line C in Fig. 2. At about 7 40 of a second later, curve a drops to zero at h in Fig. 2. Since resistor 39 is grounded, the impulse of cycle voltage shown dotted in curve b of Fig. 2 is prevented from reaching diodes 45 and 38. About of a second later brush 3 grounds, shorting resistor 38 at a time corresponding to line (1 in Fig. 2. No bias voltage exists in resistor 39 at this instant, since it is still shorted out. Brush 1 opens at a time corresponding to f in Fig. 2. Brush 3 remains grounded until a time corresponding to g of Fig.2, and thus the dotted impulse of curve a is prevented from reaching diodes be varied to give any desired length of impulse in multiples of a half cycle of the input altemating current. Also, a, single half cycle duration impulse may be generated by omitting one of diodes 23 or 23 and their associated circuits.

' The bias on diodes A5 and 38 is shown in Fig. 3. Cut-ofi is removed, as described above, for an accurate and electrically timed hm of a second from h. to It. During this interval, sub-carrier signals are passed by diodes t5 and &8 into the radio modulator and transmitter to produce a synchronizing impulse to be transmitted along with the picture signals. If the synchronizing impulse is produced between picture lines, a portion of a typical modulated sub-carrier signal will be as shown in Fig. i, where n n and s s represent picture signals and 2 i represents the synchronizing signal.

The present system eliminates synchronizing signal switching transients, since the switching brushes operate, while diodes 35 and lit are at cut-ofi.

The present description has shown a system operating from a 60 cycle power line, but it will be evident that other frequencies may be used without departing from the invention.

While only a single embodiment of the present invention has been shown and described, many modifications will be apparent to those skilled in the art within the spirit and scope of the invention, as set forth in the appended claims.

What is claimed is: l

i. In a synchronizing signal generator, means for generating a series of spaced impulses, means for generating a series of complementary impulses, means for periodically removing an i-m pulse from said first series, means for removing an adjacent impulse from said second series, means for combining said first series and said second series to provide a substantially constant signal having periodic gaps, and means for generating a synchronizing signal conforming to said gaps.

2. In a synchronous signal generator, means for generating two groups of complementary periodic signals, means for reducing the instantaneous value of one of said groups to zero fo a predetermined interval, and means for starting a synchronizing impulse during said interval whereby transients are substantially eliminated from said synchronizing impulse.

3. In a facsimile synchronizing signal generator, the combination or", a source of alternating current of predetermined. frequency, a circuit for passing positive portions and cutting-oil negative portions of a current derived from said source, a circuit for passing in reversed polarity negative portions and cutting-off positive portions of said derived current, a circuit for combining both of said passed current portions to form a substantially constant current, a synchronous switch driven from said alternating current for cuttin out predetermined halt cycles of one of said pmed currents phased to close and open between half cycles or one of said passed currents to form an electrically timed synchronizing signal gap in said substantially constant current.

4. In a facsimile synchronizing signal generator, the combination of, a source of alternating current. of predetermined substantially constant frequency, a circuit for passing positive portions and cutting-oil negative portions of a current derived from said source, a circuit for passing, in reversed polarity, negative P rtions and cuttingoil' positive portions oi said derived current, a circuit for combining both oi said passed current portions to form a substantially constant current, a pair of synchronous switches operated from said alternating current for cutting-out predetermined adiacent half cycles of said passed currents phased to close and open one circuit between halt cycles or one of said passed currents and to close and open another circuit -ietween half cycles of the other of said passed currents adjacent to said first half cycles to form an electrically .timed synchronizing signal gap in said substantially constant current-equal in duration to one cycle of said alternating current.

5. In a timed impulse generator, the combination of, a source of alternating current, means for deriving from said alternating current a series of unidirectional impulses corresponding to half cycles of one polarity, means for deriving from said alternating current a second series of unidirectional impulses corresponding to half cycles of the other polarity, means for periodically cutting out an adjacent impulse of each of said series operating between impulses or each series, means to combine said impulses to form a substantially constant current interrupted for electrically determined periodic intervals to form a synchronizing gap, and means to generate a synchronizing impulse substantially equal to said 8 -D- HUGH C. RESSLER. 

